So-called superabsorbents are polymers that are able to absorb fluid in quantities corresponding to many times their own weight. They are also able to retain the fluid absorbed even when subjected to external pressure. Such polymers have found wide use in absorbent sanitary products such as diapers, sanitary napkins and the like, said polymers normally being present in particle form, such as in the form of grains, granules, flakes or fibres, and are mixed or layered with other absorbent material, typically cellulose fibres.
The effectiveness of such superabsorbents in an absorbent structure is contingent on many factors, such as where and how the superabsorbent is incorporated in the absorbent structure, its particle form and physical and chemical properties, and also the speed at which it will absorb fluids, its gel strength and its ability to retain absorbed fluids. A phenomenon referred to as gel blocking may also influence negatively the absorbency of a fibre structure that contains superabsorbents. Gel blocking is a phenomenon in which when superabsorbent particles are wetted, they form a gel which blocks the pores in the fibre structure or the voids between the particles, therewith obstructing the transportation of fluid from the wetting area to the remainder of the absorbent body, and also obstructing the transportation of fluid to all particles. Another problem is that the superabsorbent particles located in the wetting region of the absorbent structure binds the fluid in said region already at the first wetting occasion. The acquisition at the next wetting occasions is by that deteriorated. In consequence, the total absorption capacity of the absorbent body is not utilized to an optimal extent and there is a risk that fluid will leak from the article. This problem is even more acute in the case of articles that are intended to be used over long periods of time, for instance throughout the night, where wetting often occurs on several occasions.
It is known through FR-A-2,627,080 to encapsulate superabsorbent particles in a protective membrane, which will only dissolve slowly in the fluid to be absorbed, e.g. urine. The activation of the superabsorbent material is by that delayed. The encapsulated superabsorbent particles can be applied in a layer closest to the body-facing side of the absorbent core.
It is known from EP-B-0,388,120 to mix superabsorbent particles with a porous silicon dioxide powder with the intention of reducing the hygroscopicity of the superabsorbents during storage and transportation. It is reported, however, that the absorption properties are not affected by this treatment.
WO 91/04361 teaches a method of encapsulating superabsorbent particles in a protective membrane. The protective membrane is comprised of a mixture that contains a film-forming polymer and a hydrophobic crystalline substance, and is intended to protect the particles against the absorption of fluid during the manufacture of an absorption structure by means of a wet-laying process. Subsequent to drying the structure, it is necessary to destroy the casing, for instance either mechanically or thermally, with the aid of ultrasound or the like, in order to make the superabsorbent particles active as fluid absorbents.
U.S. Pat. No. 4,548,847 describes a superabsorbent which has the form of an anionic polyelectrolyte which is reversibly cross-linked with a polyvalent metal cation. Also included is a substance which when coming into contact with fluid reacts with or binds to the metal cation to form a complex compound therewith. It is not until this has taken place that the superabsorbent is activated and able to start absorbing fluid. This results in a relatively short delay in the absorption process.